Are Samyoung caps crap cant see bulging

[snake91876]

I have a samsung dlp i have discovered the power supply is only sending 150 volts to ballast board on lamp which requires 380v. The unit powers on but shuts down. I am assuming the two caps next to the plug of wire running to ballast are bad. They are Samyoung 250v 820uf caps with a plastic not aluminum top. The top of capacitor is raised some on each one kinda convex but does not look bulged like the aluminum does. What do you think should i repalce.

[tom66]

Re: Are Samyoung caps crap cant see bulging

Only 150V suggests PFC fault.
Samyoung aren't the best, but they rarely fail.

[snake91876]

Re: Are Samyoung caps crap cant see bulging

What do you mean pfc fault exactly
thanks

[mariushm]

Re: Are Samyoung caps crap cant see bulging

There's a controller chip and some other components (a pair of mosfets maybe, resisotrs, some diodes) between the bridge rectifier and the large capacitors which should boost the voltage to 350-400v. Yours obviously isn't working, since you get 150v in those capacitors.

Follow the traces from the mains and the rectifier to the 250v caps ... you have a problem between them.

[snake91876]

Re: Are Samyoung caps crap cant see bulging

i am not that familiar with component repair but i have done some capacitor replacement i can get a board used for around thirty , do you think i can fix this one pretty cheap if i give you pictures , the only thing i have is a 600v meter no fancy equipment to test.

[budm]

Re: Are Samyoung caps crap cant see bulging

"Samyoung 250v 820uf " There must be other higher voltage (450VDC) for the PFC Voltage booster circuits, unless they put those two caps in series.
To troubleshoot down to component level with SMD components, we will need good clear pictures, component spec, etc. It can be done, but real time consuming.

[snake91876]

Re: Are Samyoung caps crap cant see bulging

Alright here are the pics and i see a potential area of problem that has darkened the board it is right beside the two large capacitors that say 250v. Let me know what you think. i also took a pic of the back side and the solder in that area looks a little strange like it was real hot , i dont know.

[budm]

Re: Are Samyoung caps crap cant see bulging

This power supply does not have PFC Voltage booster, it uses voltage double circuit, so if you read the voltage on each two pins of the cap you will see about 160~170VDC on each cap, so the total output voltage is around 320~340VDC.

[snake91876]

Re: Are Samyoung caps crap cant see bulging

well im only getting 150v total output at the plugin that takes the power to the lamp ballast. it says on the board next to the plug 380v to ballast. what could be the reason. are you telling me to check the cap voltages while its on.

[budm]

Re: Are Samyoung caps crap cant see bulging

That voltage should be there when you apply 120VAC to the board. So what voltage do you get on each cap? You need to check the bridge rectifier and the value of the caps.

[snake91876]

Re: Are Samyoung caps crap cant see bulging

with the 120vac applied to the board i had 125 on one cap and 113 on the other one, after about two minutes i had 122 on the one and 110 on the other one.

[budm]

Re: Are Samyoung caps crap cant see bulging

"well im only getting 150v" What do you use for the Ground ref point?
Tell me what you get at the B+ and the ground as I indicated, put the meter at those tow points.

[snake91876]

Re: Are Samyoung caps crap cant see bulging

Testing from those points i got about 280v but it steadily falls about a volt a second. I unplugged 120vac and tested again about 285v but steadily falls a volt a second .

[Clydeuk]

Re: Are Samyoung caps crap cant see bulging

I've encountered quite a lot of people that make the same mistake of using the chassis as a ground when testing the hot side. I guess it's a common mistake, in the old days of CRT sets here you would know about it quickly if you tried to ground your earthed scope probe on the chassis.

That scorched area is probably fine, those 3 resistors are probably high value resistors in series dropping the mains derived DC down to provide a supply for the SMPS controller. They tend to run hot and scorch the board but are unlikely to be at fault as you wouldn't have any secondary outputs otherwise.

[Clydeuk]

Re: Are Samyoung caps crap cant see bulging

You mean with the 120v hooked up the voltage still drops? Can you hear a relay drop out as it appears there is one there supplying the bridge? Could be detecting an error and killing the supply.

[snake91876]

Re: Are Samyoung caps crap cant see bulging

Nope the meter just falls slowly one volt at the time it dropped all the way down to 260s or 250s but then i unplugged and reenergized it and testing again and got back in the 280s and just falls.

[Clydeuk]

Re: Are Samyoung caps crap cant see bulging

Check back from those points to the bridge rectifier (the points you're measuring connect to the top and bottom terminals just to the left of the capacitors). Very carefully put meter on AC volts and probes on the centre two terminals on the bridge (may be better to put one probe further along the trace somewhere in case you slip with the probes, easily done).

With the AC connected you should have steady 120V AC there, if it's there for a second then drops your supply is being cut off.

[snake91876]

Re: Are Samyoung caps crap cant see bulging

could you show me a picture are you saying test the center two legs of bridge rectifier on ac volts or to test from the capacitors back to the bridge rectifier thing.

[Clydeuk]

Re: Are Samyoung caps crap cant see bulging

Got to say I'm not familiar with that design it's difficult trying to follow it from the print side only, I'm so used to PFC's now it seems strange. Maybe it would help to see what components are where on the top side, i'm a bit too tired to look right now

[Clydeuk]

Re: Are Samyoung caps crap cant see bulging

From a glance at the bottom the centre two legs of the bridge rectifier.